1. Field of the Invention
This invention relates to europium-activated alkaline earth mixed halide x-ray phosphors. More particularly it relates to barium fluorochloride and barium-strontium fluorochloride activated by europium.
2. Prior Art
Europium-activated barium fluorochloride and barium-strontium fluorochloride are known x-ray phosphors. One of the important properties of any x-ray phosphor is to produce bright light within the spectral range of the film with which it is being used. Most research work heretofore has been concentrated on improving the brightness of the emitted light in order that the time of exposure to x-rays is decreased.
One of the other important characteristics of x-ray phosphor is the time that the phosphor continues to glow after the exposure to x-rays has ceased (afterglow). Ideally there should be an instanteous response, however, this is never achieved. As can be appreciated, continued emission after x-ray bombardment has ceased may cause "ghost" image upon the subsequent film.
Additives have been used in calcium tungstate x-ray phosphors to reduce afterglow. Vanadium addition was found to be effective. An additive used to reduce afterglow cannot significantly decrease the brightness of the emission nor can it significantly decrease the power of the host to stop x-rays.
None of the known prior art has given any guidance to the problems associated with afterglow in the europium-activated alkaline earth mixed halide system. As used herein the term "alkaline earth mixed halide" refers to a host represented by the formula: EQU Ba.sub.1-x Sr.sub.x FCl
wherein x is from 0 to about 0.3. Also within the context of this disclosure, the activator is europium. While divalent europium can be used alone, in our U.S. patent application Ser. No. 698,058, filed June 21, 1976, we have shown that the presence of some trivalent europium is beneficial. Thus, the europium activator can be divalent europium of mixtures of divalent and trivalent europium as taught in the aforementioned application which is incorporated by reference herein.
It is believed that an alkaline earth mixed halide activated by europium having a significantly reduced afterglow would be an advancement in the art.